This invention relates generally to diagnostic imaging methods and apparatus, and more particularly, to methods and apparatus that provide for thermal gain management and correction in the detector for computed tomography in particular and in medical imaging in general.
Most multi-slices computed tomography (CT) scanners are built with detectors composed of scintillator/photodiodes arrays. The photodiodes arrays are mainly based on front-illuminated technology. However, new designs based on back-illuminated photodiodes (backlit diodes) are being investigated for CT machines to overcome the challenge of the higher number of runs and connections required. Current CT detectors generally use scintillation crystal/photodiode arrays, where the scintillation crystal absorbs x-rays and converts the absorbed energy into visible light. A photodiode is used to convert the light to an electric current. The electric current is read and the reading is proportional to the total energy absorbed.
A CT detector should meet tight performance requirements in order to enable the generation of high quality and artifact free CT images. First, the detector should provide a response that is linearly related to x-ray intensity. Some of the requirements on the detector that result from this are the stability of the detector over time and temperature, the non-sensitivity to focal spot motion, and a bound on the light output variation over life, etc. In a third generation CT scanner, the relative behavior of adjacent channels should be nearly identical in order to avoid serious ring artifacts (usually defined as channel to channel non-linearity variation) in images. This variation might be affected by the scintillator behavior from one pixel to its neighbor, by the collimator plate variations, and/or by the diode pixel response. Generally, if these requirements are not met, ring artifacts, bands and/or smudges/spots might appear in images.
One of the contributors of this channel to channel variation (or module to module variation) is the gain variation caused between photodiode pixels due to the variation of temperature. In Volume CT, with a relatively large coverage of the collimator, the variation of the temperature at the diode will be high and tight thermal control from calibration conditions to scanning conditions would be required. The thermal gain temperature coefficient drift in the module may have multiple root causes: a) diode, b) collimator, c) scintillator, d) DAS electronics and finally e) DAS assembly. To overcome this problem, one can either introduce very tight thermal control on the detector or compensate for the thermal drift by introducing a thermal calibration or correction. For this, an accurate measurement of the temperature on each pixel would be desirable.